The present invention relates to a process for preparing pellets of hydrolyzed ethylene-vinyl acetate copolymers, and more particularly to a process for preparing hydrolyzed ethylene-vinyl acetate copolymer pellets which have excellent thermal stability and moldability and from which impurities such as sodium acetate can be efficiently removed.
Hydrolyzed ethylene-vinyl acetate copolymers are widely employed for various uses such as films, sheets, containers and fibers since the hydrolyzed copolymers are excellent in properties such as oxygen-barrier property, solvent resistance and mechanical strength.
Generally, hydrolyzed ethylene-vinyl acetate copolymers are prepared by hydrolyzing ethylene-vinyl acetate copolymers with alkali catalysts such as sodium hydroxide. After hydrolysis reaction, a large portion of the alkali catalysts such as sodium hydroxide are converted into alkali metal salts of acetic acid such as sodium acetate, and the salts are included in resins deposited or coagulated from the hydrolysis reaction mixtures.
The impurities such as sodium acetate affect adversely properties of hydrolyzed copolymers, e.g. the coloration of the hydrolyzed copolymer occurs, the moldability lowers, and the like. Therefore, it is necessary to remove impurities such as sodium acetate from hydrolyzed copolymers as far as possible.
On the other hand, when molded articles such as films and sheets are prepared from hydrolyzed copolymers by melt-molding, the hydrolyzed copolymers are required to be formed into pellets from the point of handling or molding processability. Accordingly, it is desired that the hydrolyzed copolymer pellets having the remarkably decreased contents of the alkali metal salts of acetic acid and having excellent thermal stability and moldability are prepared and supplied into the market.
For instance, Japanese Examined Patent Publication No. 38634/1972 discloses a process for preparing pellets of hydrolyzed ethylene-vinyl acetate copolymers which comprises extruding hydrolyzed ethylene-vinyl acetate copolymers in the mixture of water and methanol into coagulating liquids of water or the mixture of water and methanol in the form of strands, and cutting the strand into pellets.
The above-mentioned process has merits that porous pellets from which sodium acetate can be removed to small amount by washing them with water can be obtained and the loss of resin in the preparation of pellets is relatively small, but the process has the following deffects.
(1) A large amount of wash water is required for removing sodium acetate from the pellets to such an extent that the residual sodium acetate does not affect adversely the thermal stability of hydrolyzed copolymers in molding it, since porosity of the pellets are not so high. Moreover, there is room for improvement from the point of the productivity, for instance, since it takes a long time to wash the pellets, the rate of desalting is slow, it takes a long time to dry the washed pellets, and the like.
(2) The residence time of the strands in coagulating liquids is long and the requirements for apparatus becomes large, since the coagulating rate of the strands extruded into coagulating liquids is slow.
(3) When the strands are cut into pellets, the form of the obtained pellets is non-uniform or the pellets are deformed, since the coagulated strands are not sufficient in hardness.
An object of the present invention is to provide a process for preparing pellets of hydrolyzed ethylene-vinyl acetate copolymers solving the above-mentioned defects.
The above and other objects of the present invention will become apparent from the description hereinafter.